The present invention relates to the field of connections in BGA, or xe2x80x9cBall Grid Arrayxe2x80x9d packages in printed circuit cards, and more particularly concerns PGBA, or xe2x80x9cPlastic BGAxe2x80x9d packages.
The standard xe2x80x9ccavity downxe2x80x9d type of PBGA package comprises a cavity in which an integrated circuit, also called a xe2x80x9cchip,xe2x80x9d is housed.
In this type of package, the areas for connecting the chip to the package and the areas for connecting an array of solder balls to the package are disposed on the same level of the package, generally corresponding to the bottom surface of the package, i.e. the surface that faces the printed circuit card when the package is mounted on the card.
The ball array connects the chip to the printed circuit card both electrically and mechanically.
In order to protect the active surface of the chip constituted by a semiconductor layer, and the connections, for example bonding wires, both mechanically and from the environment, an encapsulating material is used, for example a thermosetting and electrically neutral resin, which coats the outside of the chip and its connections.
Such a method is specifically described in U.S. Pat. No. 5,397,921A.
This method, applied to the BGA package described above, has the drawback that the height of the wires thus coated by the resin must be compatible with the height of the balls, which is typically on the order of 0.6 mm for a pitch between the balls on the order of 1.27 mm.
This imposes the near exclusive use of the xe2x80x9cgoldxe2x80x9d, or xe2x80x9cball bondingxe2x80x9d technique, and the package must include an xe2x80x9celectrolytic goldxe2x80x9d metallization, requiring the use of xe2x80x9cfeeders,xe2x80x9d which after metallization leave remaining wire ends that are equivalent to antennas for high frequencies, particularly for clock signals.
In addition to controlling the wiring loops of the wires, it is also necessary to control the height of the coating so that the wires are completely covered.
In general, a line of highly viscous adhesive is deposited around the areas for connecting the wires to the package in order to serve as a wall for retaining the coating resin, which by contrast is very fluid in nature, so that it properly coats the wires and the chip.
This wall also makes it possible to prevent an overflow of the coating resin in the areas for connecting the balls to the package.
Another drawback relates to the phenomenon of the xe2x80x9ccollapsexe2x80x9d of the balls when the package is assembled to the printed circuit card.
In essence, during assembly, the superficial tensile forces of the alloy constituting the balls, typically remelted tin-lead, causes the balls to collapse onto the card.
Thus, a ball typically having a height of 0.6 mm for a pitch of 1.27 mm and an initial diameter before assembly of 0.76 mm will decrease in height by about 0.2 mm. This phenomenon is amplified, particularly when an integrated heat sink covers the top surface of the package; this can result in short-circuiting between the balls.
The specific object of the invention is to eliminate the aforementioned drawbacks.
To this end, the subject of the invention is a package for an integrated circuit of the type comprising a cavity in which the integrated circuit is mounted, the active surface of the integrated circuit being electrically connected to the package on the level of connection of a ball array to the package, providing a mechanical and electrical link between the integrated circuit and a printed circuit card to which the package must be assembled. The package is characterized in that it comprises an additional layer that is rigid and electrically neutral, attached to the level of connection of the integrated circuit and the balls and containing the balls.
The invention has the particular advantage of simultaneously facilitating the mounting of the balls onto the package, also called the xe2x80x9cball bondingxe2x80x9d of the package, and of protecting the connections and the card.
It also makes it possible to guarantee a predefined distance of the package from the card, thus playing the role of a spacer between the package and the card.